1. Field of the Invention
The present invention relates to apparatus for simultaneously increasing the frequency difference and frequency difference stability of the two orthogonally polarized components of the beam from a Zeeman split laser. More particularly, the invention relates to an apparatus which is useful in a variety of interferometric optical measuring devices, which perform extremely accurate measurement of changes in optical path length at high slew rates.
2. Description of Related Art
The use of optical interferometry to measure changes in either length, distance or optical path has grown significantly due not only to technological advances in lasers, photodetectors and microelectronics but also due to an ever increasing demand for high precision and high accuracy measurements. Interferometers can be generally categorized into two types based on the signal processing used, i.e., either homodyne or heterodyne. The interferometers based on heterodyne techniques are generally preferred because they are insensitive to low frequency drift and noise and they can more readily have their resolution extended. Within the heterodyne type of interferometers of particular interest are the ones based on the use of two optical frequencies. The two optical frequencies are produced by one of the following techniques: (1) use of two longitudinal modes of a randomly polarized laser, see for example, J. B. Ferguson et al., "Single mode collapse in 6328.ANG. He--Ne lasers", Applied Optics, vol. 17, pp. 2924-2929 (1978); (2) use of Bragg cells, see for example, Y. Ohtsuka et al., "Two-frequency laser interferometer for small displacement measurements in a low frequency range", Applied Optics, vol. 18, pp. 219-224 (1979); N. Massie et al., "Measuring laser flow fields with a 64-channel heterodyne interferometer", Applied Optics, vol. 22, pp. 2141-2151 (1983); and Sommargren, U.S. Pat. No. 4,684,828 issued Aug. 4, 1987; (3) use of a Zeeman split laser, see for example, Bagley et al., U.S. Pat. No. 3,458,259 issued Jul. 29, 1969; Hewlett-Packard Journal (August 1970); Bagley et al., U.S. Pat. No. 3,656,853 issued Apr. 18, 1972; and Hewlett-Packard Journal (April 1983).
The use of two longitudinal modes of a randomly polarized laser provides a laser beam with two orthogonally polarized frequencies in a rather convenient, cost-effective form. However, the frequency difference is approximately 600-1000 MHz which requires complicated, expensive detection and processing electronics. Furthermore, by starting with such a high frequency difference the task of resolution extension becomes difficult and expensive.
Bragg cells have been used to produce the two optical frequencies. However, the use of Bragg cells requires a complex, expensive apparatus which is susceptible to a number of sources of error and alignment difficulties.
A Zeeman split laser is the most elegant device to produce the two optical frequencies. To date practical considerations have limited this technique to a maximum frequency difference between the two optical frequencies to about 3 MHz. This imposes a limit on the maximum rate of change (slew rate) of the length, distance or optical path being measured. Another limitation of the Zeeman split laser is that the frequency difference is sensitive to the stability of externally applied magnetic fields which can very with time, location and orientation (e.g., the earth's magnetic field or magnetic fields produced by electrical currents in nearby electrical equipment). Instability of the difference frequency can degrade the accuracy of any measuring device which relies on the constancy of the difference frequency. Further, current Zeeman split lasers sacrifice laser power in order to achieve progressively higher difference frequencies.
While current techniques for producing two optical frequencies of orthogonal polarizations are useful for some applications, none provide the technical performance in a commercially viable form for applications requiring the measurement of rapidly changing lengths, distances or optical paths to extremely high resolution.
There is a need for a cost effective laser with two orthogonally polarized beam components that can be set to the predetermined value which can be significantly higher than the difference frequency from currently available Zeeman split lasers.